High fidelity radio receiver



April ll, 1939. A, P. MONTGOMERY 2,153,583

HIGHQFIDELITY RADIO RECEIVER Filed May 8, 1935 a rrqys,

Patented Apr. 11, 1939 UNITED STATES iPTENT OFFiCE HIGH FIDELITY RADIO RECEIVER Application May 8, 1935, Serial No. 20,473 In Great Britain May 14, 1934 7 Claims.

This invention relates to radio receivers and, more particularly, to superheterodyne receivers. The principal object of the invention is to provide a high fidelity receiver of this class by the provision of means for broadening the fidelity curve or response characteristic of the receiver to give a desired substantially rectangular response curve and eliminate side-band cutting. As is well known, the response curve of the modern receiver has a pronounced resonance peak and such receiver tends to cut the side bands of the modulated carrier wave being received. It has long been recognized as being desirable to obtain high selectivity but, at the same time, maintain high fidelity so that signals of all frequencies throughout the modulated carrier band will be received at maximum even intensity.

In the commonly known superheterodyne receiver, the intermediate stages are designed so as to have a fixed resonant frequency of the desired value. In one or more of these stages, there is provided a resistor shunted by a condenser in the cathode lead for the purpose of biasing the grid of the tube. It is customary also to employ a tuning indicator or current meter which is usually connected in the output circuit of an intermediate frequency amplifier tube.

By this invention the desired fidelity characteristic is obtained by providing in one or more of the inductively coupled stages a tertiary circuit resonant to the tuned intermediate frequency and. comprising a variable resistor and fixed condenser in circuit with a coil which is inductively related to the usual coupling coils. In addition, one or more of the above-mentioned grid-bias resistors may be made variable. When the resistance in the tertiary circuit is reduced, the energy traversing it is increased, the band Width of the stage is widened and an approach 40 to the desired rectangular response curve obtained. In other words, by increasing the absorption at the intermediate carrier frequency in an intermediate frequency transformer the band of frequencies which it will pass with substantially the same amplitude may be increased and consequently the fidelity or audio frequency range of the receiver may be increased. However, by increasing the absorption in this transformer, the gain or amplification at the intermediate frequency, due to the transformer, will be reduced. This may be compensated for by increasing the gain in the vacuum tube associated therewith or any other vacuum tube in the amplifier, for instance, by reducing the value of its grid-bias resistor. The fidelity control resistor and gain control resistor are preferably adapted for joint or simultaneous manual operation. By proper selection of values for the two resistors, the over-allg-ain may be made uniform. Thus it will be seen that the invention provides means by which the fidelity or response characteristics of a radio receiver may be varied without changing the over-all gain of the receiver. It will be further noted that the fidelity characteristics may be varied continuously and noiseless-1y which is highly desirable, and that the response adjustment may extend to a point where the response is considerably sharper than would be useful were the response not adjustable, for reducing interference under unusually severe conditions.

A further feature of the invention consists in the provision of a novel tuning indicator circuit connected to the main circuit so as to derive its energization therefrom. This branch circuit may be connected to the main receiver circuit between the intermediate frequency stages and the second detector. It preferably comprises a sharply tuned circuit connected to the input of a gridleak or other detector with the tuning indicator or meter connected in the output circuit of the said detector.

The above objects and features of the invention may be more clearly understood from the following detailed description in connection with the accompanying drawing, the single figure of which is a diagrammatic illustration showing the several features applied to a superheterodyne receiver.

Referring to the drawing, there is shown an intermediate frequency amplifier IF such as commonly employed in the intermediate frequency stages of the superheterodyne receiver and while only one such amplifier is shown, it will be understood that the receiving system may comprise a plurality of intermediate frequency stages, as is well known. The intermediate frequency amplifier shown is coupled to a detector D, which constitutes the usual second detector of the system, by means of the inductively associated transformer coils P and S. The detector D is coupled to an audio-frequency amplifier AF in conventional manner, it being understood that there may be a plurality of audio frequency amplifier stages as is well known. Automatic volume control may be embodied in the system in any manner, for example by deriving a control voltage from the leak resistance of the detector D and applying such voltage through a filter resistor F to the control grid of amplifier IF as described and claimed in co-pending application Serial No. 596,599, filed; Mar. 3, 1932, Patent No. 2,086,465, dated July 6, 1937.

In accordance with the present invention, the grid biasing resistor R, which is interposed in the cathode lead of the intermediate frequency amplifier as is customary, is made variable and there is also provided a series circuit A which includes a variable resistor R1. Preferably, the variable resistors R and R1 are adapted for joint or simultaneous manual operation as indicated by the broken-line representation. The arrangement is such that the resistances decrease or increase simultaneously depending upon the direction of actuation of the control means. The coils P and S may be substantially critically, or less than critically coupled with respect to each other. The series circuit A may include a condenser C and inductance coil L which may be preferably over-coupled with respect to S. It will be understood that other degrees of coupling may be used and the invention is not intended to be limited to these particular values. The circuit A is resonant to the tuned intermediate frequency as above stated. The operation of the IF transformer including the fidelity control is as follows: For a very large Value of R1 which means that the circuit A is practically open circuited, the IF transformer, which includes the tuned primary P and tuned secondary S, operates as a conventional critically coupled tuned transformer and has the usual sharp response characteristic of such a circuit, that is, a high response at the resonant frequency which falls off rapidly on each side for frequencies slightly different from the resonant frequency. 'Said lastnamed frequencies correspondto the side bands of a modulated carrier wave as. is well known in the art. As is well known, the sharpness of the response in this case is-determined by the degree of coupling and by using slightly less than critical coupling very high selectivity may be obtained. Thus it will be seen that side-band cutting will occur and only side bands corresponding to lower audio frequencies will be transferred effectively through the transformer, and such a device will be characterized by poor fidelity. However, following the practice of the invention, the tertiary circuit A which is tuned to the resonant frequency of the transformer when R1 is zero may be used to broaden the range of side bands transmitted by the device with substantially uniform amplitude. When R1 is reduced in value the energy traversing the circuit A at the resonant frequency will increase but the amount of energy thus removed from the transformer will fall off rapidly for side bands having frequencies slightly off resonance, and at frequencies removed from resonance, the energy traversing circuit A will be returned to the transformer to increase the response. Thus the circuit A increases the fidelity range of the transformer by reducing the gain of the transformer for the carrier and low frequency side bands and increasing the gain for the higher frequency side bands. The frequency-selective loss of carrier frequency gain thus incurred is compensated for by increasing the gain of a vacuum tube amplifier as described hereinbefore, which uniformly increases the gain for all frequencies in the transmitted band thereby making the over-all gain uniform as the width of side bands transmitted is varied. Of course, for smaller values of R1, moreenergy traverses circuit A and the band width of the transmitted frequencies becomes wider. It will be understood that as the resisance is varied the resonant frequency of circuit A will be slightly modified.

As the value of the resistor R1 is decreased, causing an increase in the fidelity response of the receiver and a decrease in gain for the IF transformer, the value of R is similarly reduced, causing greater amplification in the stage IF. The two resistors may be designed so as to properly effect this compensation. In this manner, the receiver may be caused to operate along a fidelity response curve having a fiat top and including all of the frequencies of the modulated carrier band, and will have the same output intensity for any chosen band width setting.

It will be understood, of course, that the features in question may be applied to as many stages of the system as may be desired. That is to say, there may be a plurality of the resistors R in the various intermediate frequency stages or other means for adjusting the intensity, and there may be a plurality of the response curve adjusting circuits A between the various stages, with some or all of the resistors R and R1 gang controlled.

In further accordance with the invention, there is provided a branch circuit B, which may be connected to the main circuit between the intermediate frequency stage IF and the detector D as illustrated. This circuit preferably takes the form of a sharply tuned circuit which has connected thereto a rectifier E, preferably in the form of a grid-leak detector as illustrated. In the output circuit of the detector E, there is provided a tuning indicator or meter M, and. a suitable filter condenser C1.

In operation, the branch circuit B derives a portion of the incoming energy and such energy is detected or rectified by the grid-leak detector E in the usual manner of operation of such a device. The output current of this detector flow- 'ing through the indicator or meter M varies in accordance with the tuning of the receiver and the meter therefore indicates the proper tuning of the system. With the receiver adjusted for high fidelity reception, it is diffi'cult to properly tune it by ear due tothe fact that even though the receiver is improperly tuned, reception will nevertheless take place but at a loss of quality. It will be noted that the control energy for the automatic volume control is derived from the signal channel of the receiver proper and therefore the automatic volume control voltage, and.

consequently the receiver gain, will not vary even properly tuned. Thus the meter indicates at all times the correct tuning of the receiver, even when the correct tuning cannot be determined conveniently by ear.

Only such portion of the receiving system as is necessary to illustrate a simple embodiment of the invention has been illustrated, it being unnecessary to illustrate or describe the conventional receiver in further detail. It will be understood that various modifications, including those above mentioned'may be made without departing from the invention.

I'claim:

1. In a superheterodyne radio receiver having amplifying stages, adjustable means interposed in the intermediate frequency portion of the receiver for varying the frequency response of the receiver, wherein the output level is reduced as the frequency response is broadened, and means comprising a variable grid bias resistor jointly controllable with said first means for varying the gain of at least one of said stages for maintaining the output level constant.

2. In a superheterodyne radio receiver having an intermediate frequency amplifier, an interstage coupling device having adjustable means for varying the frequency response characteristic of the receiver and comprising at least one circuit tuned to the intermediate frequency, and a second tuned circuit coupled thereto and including an adjustable resistance for absorbing energy, whereby the output level is reduced as the frequency response is broadened, and means jointly controllable with said first means for varying the gain of said amplifier to maintain the output level constant.

3. In a radio receiver, an inter-stage coupling device comprising a primary circuit and a secondary circuit coupled together to form a signal transfer path, means for supplying signal energy to said primary circuit, means for deriving an output signal from said secondary circuit, said circuits being individually tuned to the same frequency and the coupling being less than critical coupling and thereby causing said signal path to have a sharp selectivity characteristic, and frequency-selective energy-absorbing means for decreasing the selectivity of said transfer path by selectively absorbing energy therefrom, comprising a third circuit tuned to said frequency and coupled to one of said circuits in said path and including a variable energy-dissipating resistance.

4. In a superheterodyne radio receiver having an intermediate frequency amplifier, an interstage coupling device having adjustable means for varying the frequency response characteristic of the receiver and comprising at least one circuit tuned to the intermediate frequency, and a second tuned circuit coupled thereto and including an adjustable resistance for absorbing energy, whereby the output level is reduced as the frequency response is broadened, and means comprising a variable grid bias resistor jointly controllable with said first means for varying the gain of said amplifier to maintain the output level constant.

5. In a radio receiver, an inter-stage coupling device comprising a primary circuit and a secondary circuit coupled together to form a signal transfer path, means for supplying signal energy to said primary circuit, means for deriving an output signal from said secondary circuit, said circuits being individually tuned to the same frequency and the coupling being not greater than critical coupling and thereby causing said signal path to have a sharp selectivity characteristic, and frequency-selective energy-absorbing means for decreasing the selectivity of said transfer path by selectively absorbing energy therefrom, comprising a third circuit tuned to said frequency and coupled to one of said circuits in said path and including a variable energy-dissipating resistance.

6. In a radio receiver, an inter-stage coupling device comprising a primary circuit having a primary winding and a secondary circuit having a secondary winding, said windings being coupled together to form a signal transfer path, means for supplying signal energy to said primary circuit, means for deriving an output signal from said secondary circuit, said circuits being individually tuned to the same frequency and the coupling being less than critical coupling and thereby causing said signal path to have a sharp selectivity characteristic, and frequency-selective energy-absorbing means for decreasing the selectivity of said transfer path by selectively absorbing energy therefrom, comprising a third circuit tuned to said frequency and having a winding coupled to one of the aforesaid windings and including a variable energy-dissipating resistance.

7. In a superheterodyne radio receiver, an intermediate frequency transformer comprising a primary circuit including a primary winding, a secondary circuit including a secondary winding coupled to said primary winding, condensers in each of said circuits for tuning said circuits individually to the intermediate frequency, the mutual coupling between said windings being less than critical coupling thereby causing said signal path to have a sharp selectivity characteristic, means for supplying signal energy to said primary winding, means for deriving an output signal from said secondary winding, and frequencyselective energy-absorbing means comprising a resonant circuit associated with one of said windings, said last-named circuit being resonant to the intermediate frequency and including a variable energy-dissipating resistance.

ANDREW P. MONTGOMERY. 

